Structure And Lattice Vibration Properties Of Deformed Carbon Nanotubes

Since their identification, carbon nanotubes (CNTs) have become a hotspot and frontier of material research and condensed matter physics. Lattice vibration properties of CNTs are of considerable importance not only in the basic interest in the phonons but also in a number of nanomechanical devices. Once the tube structure transformed, the phonon vibration characteristics will be certainly affected. In this thesis, phonon dispersion relations and Raman-active modes of radial- and axial-deformed CNTs are theoretically investigated, using molecular-dynamics (MD) simulations and the force-constant model combining with group theory calculations. The results catch on the intrinsic properties of Raman-active modes for deformed CNTs, and clarify the physical essence of Raman scattering experiments.The important results obtained are summed up as following:1. The expression of force constant is modified, from which the force constant model under constant pressure is developed. Adopting this method, the effects of hydrostatic pressure on Raman-active radial breathing mode (RBM) are systemically investigated. It has been found that the RBM transition occurs at certain critical pressure, where the tube undergoes structural transition from circle to oval shape in the cross section. The results indicate that the RBM transition can be proved as a sign of the structural transition of CNTs. Furthermore, the dependence of the RBM transition pressure on tube diameters is achieved, i.e., P~1/d3, which provides the theoretical foundation for experimental determination of nanotube diameters and species.2. On the basis of the correlation between different point groups, it is indicated that the deformed RBMs are still Raman-active above the critical pressure. It means that the RBMs still exist under higher pressure. Furthermore, it is shown that, above the critical pressure, the Raman intensity of the RBMs becomes too weak to be experimentally detected, which may help us clarify the essence of the experimental observations.3. Aiming at the controversial results about shifting rate of G band in Raman experiments, pressure effects on Raman-active tangential stretching modes (TSMs) are mainly studied. It has been found that the TSMs present anomalous pressure behaviors near the critical pressure, i.e. the shifting rate of TSMs showing an increased value, a constant value, or a negative value. The results consumedly help us understand the nature of the experimental G band for CNTs.4. Due to the experimental difficulty of compressing CNTs under axial stress, the effects of axial compression on RBM and TSMs are systemically studied. Under larger compression, a sharp reduction of the RBM frequency is found at certain critical strain, where the tube buckling deformation occurs exactly. Moreover, the critical strain of the RBM is inversely proportional to the tube diameter, and also depends on the tube chirality, which predicts new phenomena of Raman experiments. Furthermore, three TSMs present two kinds of different slopes with axial strain, which can be assigned to the experimentally measured G+ and G- peaks. The results are valuable to understand the effect of axial strain on tubes under non-hydrostatic pressure.